Disc drives are the primary data storage devices employed for mass storage of computer programs and data used in computer systems. Disc drives typically use rigid discs, which are coated with a magnetizable medium for storage of digital information in a plurality of circular, concentric data tracks. Read and write heads are adapted to read information from and write information to the data tracks.
The read and write heads are carried by a slider which is connected to an actuator mechanism through a gimbaled attachment. The actuator mechanism moves the slider from track-to-track across the surface of the disc under control of electronic circuitry. The actuator mechanism includes a suspension assembly that applies a load force to the slider to urge the slider toward the disc. As the disc rotates, air is dragged and compressed under an air bearing surface (ABS) of the slider that creates a hydrodynamic lifting force, which counteracts the load force and causes the slider to lift and “fly” in close proximity to the disc surface. The gimbaled attachment between the slider and the suspension assembly allows the slider to pitch and roll as it follows the topography of the disc. During rotation of the disc the write head writes magnetic bits of information (signal fields) to the disc and the read head senses the magnetic bits (signal fields) from the disc. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A magnetic gap is formed between the first and second pole piece layers by a write gap layer at an air bearing surface (ABS) of the write head. The pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field across the gap between the pole pieces. This field fringes across the gap at the ABS for the purpose of writing the aforementioned magnetic bits in circular tracks on the rotating disc. It should be noted that both longitudinal and perpendicular recording methods are utilized for writing information on disc surfaces.
A typical sensor employed by recent read heads for sensing signal fields from the rotating magnetic disc is a spin valve sensor. The spin valve sensor includes a nonmagnetic spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the ABS and the magnetic moment of the free layer is located parallel to the ABS but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer. The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered in phase by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel scattering is at a minimum and when the magnetizations of the pinned and free layers are antiparallel, scattering is at a maximum. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. When a sense current is conducted through the spin valve sensor in a direction parallel to surface planes of the layers resistance changes cause potential changes that are detected and processed as playback signals by the processing circuitry.
Another type of sensor is a tunnel junction sensor which receives a tunneling current perpendicular to the surface planes of the layers. The tunneling junction sensor includes a nonmagnetic nonconductive spacer layer between a ferromagnetic reference layer and a ferromagnetic free layer. The spacer layer, which may be an oxide, is thin enough that electron tunneling occurs between the free and reference layers. The resistance of the sensor is spin dependent, which means that the resistance of the sensor changes as a function of the relative orientation of the magnetic moments of the free and pinned layers. The pinned layer is located on and exchange coupled to an antiferromagnetic pinning layer, which pins a magnetic moment of the pinned layer in a first direction, which is typically perpendicular to the ABS. The free layer has a magnetic moment, which is free to rotate in response to signal fields from the rotating disc. A tunneling current tunnels through the spacer layer. When the magnetic moments of the free and reference layers are parallel the resistance to the tunneling current is at a minimum, and when these moments are antiparallel the resistance to the tunneling current is at a maximum. Accordingly, as the tunneling current is conducted through the tunnel junction sensor increases and decreases in the resistance of the sensor causes potential changes that are processed by the aforementioned processing circuitry as playback signals. The processing circuitry employs these potential changes to produce readback signals.
As an ever-increasing amount of information is stored on a magnetic medium, it becomes difficult for MR sensors to separately read the stored information without also reading noise from adjacent stored information. Thus, as areal densities increase, there has to be a corresponding increase in a sensitivity of the MR sensors. In general, the sensitivity of the above-described conventional spin valve and tunnel junction MR sensors cannot be increased easily without altering dimensions (such as thickness, cross-track width, etc.) of the sensor. Therefore, such sensors may encounter certain limitations when used in very high areal density applications.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.